textile design & innovations

Silk. Part II: Spider Silk

Spider Silk has been used for years for the cross hair in gunsights and microscopes. It is tenth of the diameter of human hair but it has unique mix of properties, such as flexibility, ability to absorb large forces and strength. Stopping a fly in the spider web is equal to stopping a jet in 25 mm thick filament web. This strength is considered, when compared weight to weight, five time higher than steel. The fibres are used in ballistic application, aerospace and different medical applications.

In this article we will discuss the Spider Silk production, its types and commercial applications.

Spider Silk Fibres

Spider Silk fibres is on of the world’s strongest fibres, they are light weight, flexible, waterproof and sustainable. The spider web stretch up to 40% before it breaks, this ability to absorb energy before breaking was an important focus of scientists since it is the result of the fibres ability to absorb and disperse the impact energy making it so tough.

The spider silk is derived form complex molecules of protein that is in the insects they eat. The spinning of these fibres begin with secreting the protein in gel form, this protein will harden when extruded to form the fibres. Spiders produce different types of silk for different application, such as web or egg sac, and these silk fibres differ in properties. In general, five types of spider silk shown in figure 2 below.

Figure (2): Spider Silk types

– Minor Ampullate silk (2,3), this silk is low strength and high elasticity, temporary scaffolding as the web is being built.

– Dragline silk (1), it is used to manufacture the outer rim and spokes, it is the spider lifeline.

– Capture spiral silk (5), is the sticky, stretchy fibres with glue droplets. These fibres are tough to catch the prey.

– Aciniform silk, is the silk used to wrap the prey, these fibres are three times tougher than the dragline.

-Tubiliform silk, is used to make egg sacs.

Spider Silk Production

Each spider species has a unique glands to produce it special silk. Each spider has six or seven glands that produce these different fibres.

It is worth mentioning that producing the some of the strongest fibres in the world now, such as Kevlar, Vectran and Carbon, required extensive processing with bad environmental effects, they take too much time to biodegrade and expensive recycling process. Spider Silk is the opposite of that, producing it doesn’t require any extensive processing or harms to the environment and the fibres are completely biodegradable.

It is assumed that the properties of the Spider Silk is caused by the structure of the polymer forming it. The polymer has two parts that have their own properties, one is supple, soft, and elastic, while the other is hard crystallites. In normal cases, it is very difficult to domesticate the spiders in order to cultivate silk. Spiders have cannibalistic tendencies, very aggressive and won’t produce much silk in captive.

However, scientists were able to broken the DNA code responsible for producing the Spider Silk in order to define the gene responsible. First they introduce the gene to bacteria culture to make the colonies produce the silk but it was not cost effective and did not work on large scale production. Then, when work was joint with Nexia, the gene was introduced to goats to synthesized the silk with milk production. This process allows to harvest the spider silk protein from the milk in large quantities. To produce a single vest, the daily production of 200 goat is needed.

Moreover, some other research teams are trying to introduce the gene to crops, such as alfalfa, to create more cost effective process. Other documented research is introducing the gene to silkworm to increase the production.

Applications

As mentioned earlier, the silk fibres are five times stronger than steel when compared gram for gram and they are tougher than Kevlar and much lighter (three time lighter). Thus the Spider silk find its way in different application in the military and medical application. It can be used in bullet proof vests.

The applications in medical are vast due to its natural antibiotic properties. Its strength makes it perfect for artificial ligaments and tendons.

The spinning of protein fibres form milk will be discussed later in a different article about bio-textile.